How Big Should A Koi Pond Filter Be?

If you are getting into koi keeping, you probably already know that water chemistry is a big part of keeping fish happy. And you know that having a filter that is up to removing waste from the water is essential to water quality. So, how big should a koi pond filter be?

When sizing a koi pond filter, aim for both mechanical and biological filtration to double your pond’s volume capacity. For example, a 2,000-gallon pond should have 4,000 gallons capacity for filtration. Clean water is vital to koi health, so it is difficult to over-filter.

A filter system has multiple components, and you need to size all these correctly to ensure proper filtration. Under-sizing any one element of the filtration system will seriously undermine the entire system’s efficiency, whereas over-filtration is almost impossible.

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How To Calculate The Effective Volume Of Your Koi Pond

To size your koi pond filters correctly, you will need to know the actual volume of your pond and the effective volume when you have compensated for various factors that diminish filtration effectiveness.

Calculate the volume of your pond in cubic feet by multiplying the length by breadth by depth. Irregular ponds will require an average of multiple readings.

Then find the volume in gallons by multiplying by 7,5. Irregular ponds may have a slightly lower volume in gallons than the final result indicates.

To compensate for various environmental factors, multiply the number of gallons by 1,25 for a pond shallower than 2 1/2 feet deep or if the pond is entirely in the sun. If neither of these factors applies, multiply by 1.

To compensate for climate, multiply the result you got in the previous step by 1,15 for temperate climates and 1,35 for subtropical climates. The result after this step is the effective volume of your pond in gallons. 

When sizing your pond filter, work with this effective volume.

When pumping the pond, aim to pump the entire volume of the pond in an hour. In other words, for a 2,000-gallon pond, strive to pump at 2,000 gallons per hour. You could use the actual volume of the pond, but for best results, use effective volume as explained above.

Stocking density will also affect your filtration needs. Do not stock more than one fish per 250 gallons of water.

Optimal Sizes For Components Of A Koi Pond Filtration System

A koi pond filtration system consists of several components, namely a gravity-fed bottom drain or two, a settling tank or two, mechanical filtration, and biological filtration.

The Optimal Size For The Bottom Drain In A Koi Pond

A koi pond filtration system should optimally have a bottom drain that continuously feeds the filter system.

It should be gravity feeding so that the large components of the rubbish you filter from the pond stay as intact as possible and enter the settling tank. Pumping to the filter system purees the poop and other muck, making filtration more difficult.

It should feed continuously to prevent the build-up of rubbish at the bottom of the pond. If muck builds up at the bottom of the pond, it will decompose anaerobically and release hydrogen sulfide into the bottom water.

Because koi spend the winter at the bottom of the pond, this is the worst place for hydrogen sulfide to be building up. So you want to have a bottom drain that prevents a build-up of muck at the bottom.

To do so efficiently, the diameter of the bottom drain should be a minimum of four inches. You will need to clean it out every few years as sediment gets deposited inside the pipe. But using anything smaller is inviting disaster.

A pond between 4,000 and 6,000 gallons can do fine with one sufficiently sized bottom drain going to a reasonably large settling tank; one larger than 6,000 gallons should have two separate drains and settling tanks.

Remember to cover the drain with a grid to prevent baby koi from being sucked in.

The Optimal Size For The Settling Tank In A Koi Pond

The most efficient type of settling tank is a vortex, also called a whirlpool. Although vortex filtration is less popular than other mechanical filtration methods, it is highly effective, and we recommend using it.

The water from the drain enters at an angle about two-thirds of the way down, creating a swirling motion of water in the tank that sends the larger bits of rubbish to the container’s sides. 

The container’s shape helps trap larger bits of muck, preventing them from going through to the mechanical filter.

Size the container to match the rate of flow through the pond. A larger container can handle a greater rate of flow.

A container 40 inches deep and 40 inches across will be big enough for a flow rate of 2,000-2,400 gallons per hour. A flow rate of over 2,400 gallons per hour will require a larger tank.

The Optimal Size For Filter Pipes In A Koi Pond

The pipes connecting the various components of your koi pond filter system will need to have a diameter that suits the amount of water you are pumping. 

For a 500 gallon per hour flow rate, use 2-inch diameter pipes; for over 1,200 gallons, use 3-inch diameter pipes.

The Optimal Size For The Mechanical Filter In A Koi Pond

The mechanical filter is the filtration stage where particles of waste get strained out of the water. Before biological filtration, it is essential to filter visible muck out of the water.

One can either buy or make a mechanical filter.

One way to make a filter is by creating an array of aluminum or stainless steel rods and hanging cylindrical brushes with a stainless steel core and nylon bristles in overlapping rows. Create at least four overlapping rows, and you’ll have an effective mechanical filter.

You could also use chicken grit, sponge, filter foam, or filter cotton for mechanical filtration. Either way, remember that you will have to clean it out from time to time, preferably with non-chlorinated water (to prevent killing beneficial bacteria in the biological filter).

Otherwise, there are various commercial solutions on the market. RDF (Rotating Drum Filter) is a popular option, which uses 200–300 micron-sized screens to filter debris out of the water.

Other systems use beads to filter, which resemble a pool filter. These combine mechanical filtration with some biological filtration.

Use a filter that can handle twice the capacity of your pond in gallons. So for an effective volume 2,000 gallon pond, use a filter rated for 4,000 gallons. It is tough to over-filter, and under-filtering could prove deadly for your fish, so use more filtration!

The Optimal Size For The Biological Filter In A Koi Pond

Biological filtration is the most important part of filtration. In this stage, the invisible contaminants in the water that affect its quality are broken down into harmless compounds by beneficial bacteria.

The bacteria are nitrifying bacteria, and they break ammonia and nitrites down into nitrates that plants growing in your pond can use.

If you don’t have plants growing in your koi pond itself, we recommend having a secondary pond or bog area for plants to soak up nitrates.

The nitrifying bacteria grow pretty quickly, provided you supply them with the basics they require: non-chlorinated water, oxygen (they are aerobic bacteria), and plenty of surface area to grow on. 

In time, these beneficial bacteria will not only be found in your biological filter but also on rocks and other surfaces in your koi pond.

Provide oxygen for the bacteria by installing a waterfall and an air pump. Some koi owners think that a waterfall is enough, but a good air pump can enormously increase the number of beneficial bacteria growing in your pond. 

You can build your own biological filter or buy a commercial solution. Either way, you will be purchasing the filling: material to provide lots of surface area for the nitrifying bacteria to colonize.

These can be bio balls, Japanese filter mats, brushes, or ribbons. We recommend avoiding material such as aggregate, as it is more difficult to clean (despite mechanical filtration, this filter will require cleaning occasionally).

As far as sizing goes, more is better. You can hardly have too much filtration. Follow the same rule of thumb as for mechanical filtration, and use double the capacity of your pond. 

You can also add zeolite, a mineral that absorbs ammonia. This mineral is valuable when the biological filter has not yet had a chance to develop healthy biomass of bacteria or when adding new fish to the pond.

Test regularly for ammonia and nitrites. You should detect zero contamination. If you have a non-zero reading for either contaminant, increase the amount of biological filtration.


As you can see, to do filtration right, you should always err on the side of more filtration than you think you need. More filtration means better water quality, which means happier fish.

The basic rule of thumb when sizing a 

filtration system is to make it twice as big in a capacity as the volume of your pond. Doing so ensures that you have sufficient filtration.

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